Open-loop step motor control system

ABSTRACT

A motor control system that reduces noise while reducing power requirements but providing sufficient torque. A one-phase-on drive mode modified to microstep between “one phase on” positions is used to accelerate the motor in a non-linear manner to a maximum speed. The microstep drive modes have a constant period, and the step rate of the motor is increased by decreasing the number of microsteps in succeeding steps of the motor. A modified full step waveform maintains the motor at the maximum speed, where the current to one winding of the step motor changes polarity within one motor step while the other remains approximately constant providing increased smoothness in the drive and reduced noise. The polarity changes in accordance with the inductance and the compliance voltage of the motor. A one-phase-on waveform modified to include microsteps between “one phase on” positions later decelerates the motor in a non-linear manner, and the motor stops in a detent position until another waveform of steps is delivered to the motor.

This application is a continuation of application Ser. No. 09/356,785,now U.S. Pat. No. 6,211,642, filed Jul. 20, 1999, which is acontinuation of application Ser. No. 08/526,468, filed Sep. 11, 1995,now U.S. Pat. No. 6,016,044.

BACKGROUND

The invention relates generally to motor control and more particularly,to open-loop step motor control systems that reduce acoustic noise whilemaintaining sufficient torque.

A step motor applies torque to its load in a series of discrete stepsand consequently may act as a sound transducer, generating an audibletone with a fundamental frequency equal to its step rate. If the motoris to be operable over a wide range of step rates, one or more of theserates will probably excite resonant frequencies of the motor'smechanical load, or of the motor itself, resulting in the production ofobjectionable amounts of acoustic noise and in less efficient operation.

In the medical equipment field, it is usually desirable to lower thenoise level of the equipment for the benefit of the patient and others.For example, infusion pumps containing step motors are generally locatednext to a patient and may operate for hours. It can be disturbing to apatient when the pump generates a large amount of noise. Additionally,certain medical equipment, including many infusion pumps, must bepowered by a portable power supply having a limited reservoir of power,such as batteries, and therefore the equipment must be designed toconsume as little power as possible. In this way, the equipment cansupport the patient for as long as possible before a battery change orrecharge is required. Thus, lowered levels of noise and lowered levelsof power consumption are desirable characteristics in infusion pumps andother medical equipment.

A source of acoustic noise in a step motor is the wave shape of themotor drive. The simplest means of driving a step motor is the “fullstep” mode in which a two-phase motor is driven by a current or voltagesquare wave of constant magnitude. In this mode, each step correspondsto one of 2^(N) possible motor winding current polarity states where Nis the number of motor windings (or phases). This type of drivegenerates acoustic noise with high harmonic content due to the highangular acceleration resulting from the high rate of change of torquethat occurs at the leading edge of each step. Additionally, where thedrive rate is sub-optimum and the rotor reaches its position before thewinding currents are switched, a damped oscillation of the rotor aboutthe motor magnetic field position may occur with resulting excess noiseand wasted power in providing negative torque to hold the rotor andenergy is lost in merely heating the windings due to the resistanceencountered.

The noise component can be reduced if the magnitude of the torque pulsesis decreased by reducing the magnitude of the motor drive pulse. Such areduction, however, also reduces the motor's available torque reserve,resulting in an increased risk of motor stall or “pull out” where “pullout” refers to the loss of synchronization because the load on the motorexceeds the power available to the motor to move the load, thus themotor “pulls out” of its movement cycle and loses one or more steps.This condition can result in positioning errors due to the lost steps.

Having an adequate torque reserve is necessary in the case where certainundesirable conditions may occur. In the medical field where a stepmotor is used to drive a pumping mechanism, such as a peristaltic pump,the head heights of the infusion fluid change, infusates may beparticularly viscous, and cold temperatures may require greater power tomove the peristaltic mechanism, for example. The motor's rated torqueshould be high enough to handle all of these circumstances but in anycase, its rated torque plus its torque reserve must be high enough ormotor pullout may occur. Typically, a mechanism has a rated torque and atorque reserve. In one embodiment, the reserve torque is set at seventypercent of the rated “no stall” torque.

It has been found that motor noise can be significantly reduced by thetechnique known as “microstepping.” “Microstepping” is a means ofdriving a motor through a step with a series of current magnitude statesthat generate smaller angular displacements of the motor magnetic fieldvector position. The sum of these displacements equals that of one step.Because instantaneous torque is approximately a sinusoidal function ofthe angular displacement of a motor's field vector position from itsrotor position, a smaller angular displacement results in a lowerinstantaneous torque. A lower instantaneous torque generates an angularacceleration at the leading edge of each “microstep” smaller than thatwhich would be generated at the leading edge of each step in “full step”drive mode. The effect is to spread the large acceleration that normallyoccurs at the beginning of a step over the entire step as a series ofsmall accelerations, thus reducing the level of acoustic noise.

However, “microstepping” is not a satisfactory noise reduction techniqueif power consumption must be limited, as in battery-poweredapplications. In the microstep technique, motor winding currents, thatdefine the state sequence, must be maintained throughout the sequence,resulting in relatively high power consumption. Other lower powerconsumption step modes are available, such as “one phase on” mode wherethe winding currents are turned off after the initial acceleration toconserve power. However, these modes are noisier than the microsteppingmode. Microstepping is also not desirable where controller bandwidth islimited. As the number of microsteps increases, the controller bandwidthrequirement increases requiring greater hardware capability to support afaster clock speed. This greater ability results in increased expenseand complexity.

The type of motor drive circuit can also have a direct effect onexpense. For example, closed-loop drive circuits typically requiresensors to provide the necessary feedback for control. The cost of thesensors as well as the additional processor bandwidth required to usethe sensor inputs to control the drive circuit can result in asubstantial increase in cost. An open-loop control system is preferablein this regard.

Thus, greater control over power consumption is important inapplications where long battery life is desired. Providing excessivepower to the step motor windings can cause wasted power and shortenedbattery life. Power can be lost as heat due to winding resistance.Similarly, moving the motor at its resonance frequency is inefficientand can result in wasted power because relatively little torque iscreated from the large input power that is provided to the motor. Thusprecise motor control is desirable to avoid wasting limited energy.

Hence those skilled in the art have recognized the need for lowering theacoustic output of medical devices while also lowering the powerconsumption, but retaining an adequate torque reserve. Additionally,those skilled in the art have also recognized the need for an open-loopcontrol system to reduce hardware and processor costs. The presentinvention fulfills these needs and others.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention is directed to acontrol system for controlling the movement of a motor, the systemcomprising an energy source and a controller for controlling theapplication of energy to the motor from the energy source to controlmovement of the motor, wherein the controller applies energy to themotor in a non-linear increasing manner to begin movement of the motor.In another aspect, the controller removes energy from the motor in anon-linear decaying manner to stop movement of the motor.

In more detailed aspects, the controller applies energy to the motor inan exponentially increasing manner to begin movement of the motor andremoves energy from the motor in an exponentially decaying manner tostop movement of the motor.

In further detailed aspects, the controller applies energy to the motorin multiple drive modes during acceleration from a stop, duringoperation at a constant speed, and during deceleration to a stop. Inmore detailed aspects, during the non-linear application of energy foracceleration of the motor, “one phase on” drive mode is modified tomicrostep between the “one phase on” motor step positions duringmovement of the motor. Upon reaching a desired speed, the controllerapplies energy to the motor in a full step drive mode to maintain thespeed constant. During the non-linear removal of energy for decelerationof the motor to a stop, the controller applies energy in the “one phaseon” drive mode modified to microstep between the “one phase on” motorpositions ending with the motor placed in a “one phase on” position by a“one phase on” drive mode. During periods when the motor is stopped, therotor is held in position with detent torque and no energy is applied.

In yet another aspect, the controller decreases the number of microstepsper motor step during periods of acceleration of the motor and increasesthe number of microsteps during deceleration.

In further aspects, the invention provides a motor control system for astep motor having at least two phases and a permanent magnet capable ofdefining a detent position. The control system comprises a motorcontroller providing a first and second mode drive signals to the stepmotor to accelerate the step motor in a rising non-linear manner to aconstant speed. The motor controller provides a third mode drive signalto the step motor to maintain the constant speed, and the motorcontroller provides a fourth mode drive signal to decelerate the stepmotor from the constant speed in a non-linear decaying manner to thedetent position, wherein the motor controller provides no drive signalsto the motor after the motor stops in the detent position for apredetermined period of time.

In a further aspect, the motor is continuously run through apredetermined group of steps in a time frame and is then shut off forthe remainder of the time frame to conserve power. The motor position isheld during shut off by the detent torque of its permanent magneticfield.

Other aspects and advantages of the invention will become apparent fromthe following detailed description and the accompanying drawings,illustrating by way of example the features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a basic two-phase step motor;

FIG. 2 is a block diagram of a controller for a step motor in accordancewith an aspect of the invention and the application of the controllerand step motor to the infusion of medical fluids to a patient;

FIG. 3 is a circuit diagram of the drivers and motor windings shown inFIG. 2 in accordance with an aspect of the invention;

FIG. 4 presents waveforms of a constant rate two phase step drive fordriving a two phase step motor;

FIG. 5 is a graph of the non-linear application of energy to a motor toattain a peak winding current, in this case, an exponential applicationof energy is shown and is compared to a linear application of energy;

FIG. 6 is a graph of the non-linear acceleration of a motor in responseto the exponential application of energy shown in FIG. 5, in this case,an exponential acceleration is shown and is compared to a linearacceleration;

FIG. 7 includes graphs illustrating the use of multiple drive modes incontrolling the application of energy to a step motor;

FIG. 8 illustrates in greater detail, certain waveforms of FIG. 7 inaccordance with an aspect of the invention;

FIG. 9 includes vector diagrams of the drive modes of FIG. 8;

FIG. 10 illustrates the effect of motor inductance on winding current athigh speed resulting in smooth magnetic field displacement;

FIG. 11A illustrates one of the waveforms of FIG. 10 in further detailand in FIG. 11B, a vector diagram is shown; and

FIG. 12 is a flow chart of the control of a step motor in accordancewith aspects of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, like reference numerals will be used torefer to like or corresponding elements in the different figures of thedrawings. The following discussion will be based on the illustrativeexample of a two-phase step motor 10 as shown in FIG. 1. The step motor10 includes a rotor 12 having a permanent magnet and being rotationalabout a pivot point 13, and two pairs of stator windings 14 and 16. Eachstator winding represents a phase of the step motor. For purposes ofdiscussion, the winding 14 will represent phase A, the winding 16 willrepresent phase B. The rotor 12 moves in steps in accordance with themagnitude and polarity of the current applied to the respective windings14 and 16. If a current is passed through one of the windings, theresulting north and south stator poles of the energized winding 14 willattract the south and north poles of the rotor 12, respectively.

There are a number of drive modes for controlling the rotation of therotor in a step motor. In a “one phase on” drive mode, one winding isfully energized while the other winding is turned off. By changing thecurrent flow from the first winding 14 to the other winding 16, thestator field rotates ninety degrees. Lower power is required in thismode. This results in the rotor turning a step of ninety degrees. As isknown in the art, steps of different degrees can be obtained usingdifferent rotor and stator configurations. In addition, if the two phasecurrents are unequal, the rotor will tend to shift to a position towardsthe stronger pole. The “microstep” drive mode utilizes this effect andsubdivides the basic motor step by proportioning the current applied tothe two windings. For example, by alternately energizing one winding andthen two, the rotor moves through a smaller angular displacement and thenumber of steps per revolution is doubled. Higher resolution, bettersmoothness but some loss of torque result. This mode is commonly knownas “half-step” drive mode.

For a two-phase step motor driven in “full step” drive mode, the twowindings or phases are kept energized, and the current is alternatelyreversed in each winding on alternate steps. Greater torque can beproduced under these conditions because all of the stator poles areinfluencing the motor. Their individual fields sum to produce a greatermagnetic field. However, more power is consumed in this drive modebecause both windings are constantly powered.

When there is no current flowing through the windings, the rotor willattempt to minimize the reluctance, or magnetic resistance, of itspermanent magnet by aligning itself with the poles of one of the statorwindings. The torque holding the motor in this position is referred toas the detent torque.

FIG. 2 shows a processor 20 that provides signals for driving the stepmotor 10. In this embodiment, the processor 20 or other suitable digitalsystem accesses data from look-up tables stored in a memory 22 so as toprovide signals defining the waveform for driving the step motor 10 in aparticular mode. The tables in memory 22 provide values for thepolarities and magnitudes of the currents to be applied to the windingsof the motor 10. The processor 20 supplies the polarity and magnitudesignals to the drivers 26 for providing the proper currents to thewindings of the step motor 10. The values are put through a D/Aconverter 28 to convert them to analog signals before being input to thedrivers 26.

As shown in FIG. 3, the drivers 26 for the windings 14 and 16 of thetwo-phase step motor 10 comprise a pair of H bridges 30 controlled bythe processor. The magnitudes of the H bridge current outputs arecontrolled by choppers 32. The choppers 32 act to turn the motor driveon or off as required to minimize the difference between the sense andthe magnitude signals at the comparator inputs 33. The magnitude signalsare generated via the D/A converters 28 (FIG. 2). The processor 20switches between the separate tables stored in the memory 22 thatcontain the specific data for providing the appropriate waveform todrive the step motor. The memory 22 in one embodiment contained multiplelook up tables, each of which was available to the processor for useduring operation of the motor through each group of steps. The processoris thus able to change motor drive waveforms “on the fly” andautomatically does so as described herein. The look up table index ischanged during motor rotation so that the processor always has theproper waveforms available for controlling the motor. For example, alook up table was stored for the acceleration waveforms and another wasstored for the full step waveform.

FIG. 3 also shows drive current by arrows having dashed stems and decaycurrents by arrows having solid stems 31 for the phase A winding Thechoppers 32 turn the transistors A1 and A2 on depending on the result ofthe comparison of the magnitude signal to the sense signal by thecomparator 33.

In step motors 10 used with medical infusion pumps, such as the linearperistaltic pump 34 shown in FIG. 2 acting on a tube 36 connectedbetween a fluid reservoir 38 and a patient 40, a full step drive modeshown in FIG. 4 at a constant step rate may not be the most efficientmode for operating the motor 10 to reduce noise and power consumption.As discussed above, the resulting step period can have an excessiveduration in which the majority of motion of the motor occurs near thebeginning of the period with power wasted as heat in the resistance ofthe windings 14 and 16 for the remainder of the period and objectionablenoise resulting.

Referring now to FIG. 5, an embodiment is shown in which energy isapplied to the motor in a non-linear manner to begin motor movement. Thewinding current 42 is applied in an exponential manner to cause themotor to attain its maximum torque at a rate faster than if a linearincrease in winding current 44 were used. This approach results insmoothly transitioning to a high torque output with a low initial rateof increase of torque, thereby generating less noise and consuming lessenergy than if a linear approach were used. As shown in FIG. 5, the peakwinding current 46 is attained much faster with the exponentialapplication of energy to the motor than with the linear application. Inthis embodiment, the peak winding current is applied within one motorstep. Although not shown, a like approach is used at the point ofdeceleration of the motor. The power is removed in a non-linear decayingmanner, in this embodiment, an exponential decay.

FIG. 6 presents a graph of motor movement as a result of the exponentialapplication of power shown in FIG. 5. The motor more quickly attains themaximum angular velocity 43 through exponential acceleration 45 thanthrough a linear acceleration 47. This will result in the motor passingthrough any resonance frequencies that may exist faster with less noiseresulting than if the linear approach were used. In this embodiment, themotor has attained its peak angular velocity within four motor steps.Additionally, less power is required to get to the desired speed whenaccelerating exponentially.

Referring now to FIG. 7, two time frames 48 and 50 of phase currents A51 and B 53 for a step motor drive are shown. In each time frame, themotor is moved through a predetermined group of steps 52, 54, and 56 andis then stopped for the remainder of the time frame 58. Therefore, eachtime frame includes periods of acceleration 52, maximum step rate 54,deceleration 56, and power off or stop 58 (although numerals are onlyshown on one frame). In the unpowered interval in this embodiment, therotor is held in position by the detent torque of its permanent magneticfield. It has been found that, for the same average step rate, drivingthe motor in the manner shown; i.e., non-linear acceleration to aselected maximum step rate, deceleration by a non-linear decay of steprate, and power off, results in the use of less average power to controlthe motor than the constant rate drive shown in FIG. 4.

FIG. 8 shows two winding current waveforms 51 and 53 of the group ofsteps in a single time frame of FIG. 7 in greater detail. FIG. 8 will beconsidered with the vector diagrams in FIG. 9 in the followingdiscussion. FIG. 9 contains graphs showing acceleration-deceleration andconstant speed vector sequences for the two-phase step motor 10 drivenby the waveform shown in FIG. 8. The graphs illustrate the threeportions of the waveform, and their corresponding motor steps andmicrosteps. The vectors indicate the direction and magnitude of themotor magnetic field acting on the rotor 12 at each microstep. Theacceleration portion illustrates the rapidly increasing step rate (i.e.,decreasing microsteps for successive steps) as the rotor increases speedby use of a modified “one phase on” mode. The high speed portionmaintained by the modified full-wave waveform maintains the motor'sconstant speed with accurate positioning. The deceleration portionillustrates the rapidly decreasing step rate as the motor decreasesspeed again with a modified “one phase on” mode. At microstep no. 40,the end of the deceleration portion, the rotor is near its detentposition before current to the motor windings is discontinued. The rotorstops in a “one phase on” position before the winding is turned off. Therotor is then held in position by the detent torque produced by thepermanent magnet of the rotor until the next group of steps is appliedto the motor. This results in accurate positioning of the motor withoutuse of current to hold the position.

During acceleration and deceleration, the motor is driven with amodified “one phase on” waveform. This corresponds to steps 1-4 and16-18, wherein each step begins and ends with the motor in a detentposition where the one energized winding or phase can be turned on oroff without any resulting motor torque. In the actual embodiment shown,a modified “one phase on” drive mode is used to result in smootheracceleration by the motor. The magnetic field is not precisely in thedetent position but is retarded somewhat at the end of acceleration inmotor steps 2 and 3 to prepare the rotor for the transition to highspeed drive. The field is similarly retarded slightly at the end of eachdeceleration step to prepare the rotor to coast to a stop precisely at adetent position at the moment the winding current is removed. The amountthat the second winding is energized to accomplish this modificationdepends on the physical parameters of the motor. For example, the rotormoment of inertia, the frictional loads (static, gravitational, andviscous), the torque output of the motor, the strength of the detentfield and the resistance and inductance of the motor windings all mayaffect the noise of the motor and can be considered in selecting themodification of the “one phase on” waveform.

The “one phase on” drive modes are modified to microstep between “onephase on” positions of the rotor during movement of the motor. Thetorque may be increased and a smooth field vector displacement sequenceprovided by temporarily energizing more than one winding during eachstep. Microsteps are used in the modified “one phase on” waveform tolessen the angular displacement and noise and to provide smootheracceleration for the motor. A preferred microstep sequence generates anexponentially rising current magnitude throughout the initialacceleration step as shown in FIG. 5.

As mentioned above, the motor is preferably accelerated at the beginningof the time frame at an exponentially rising step rate until the maximumstep rate for the motor is attained in a minimum time. An exponentiallyrising step rate allows the step rate to move past the resonancefrequencies of the motor more quickly, thereby reducing acoustic noise,after starting the motor from a stationary position with a low initialangular acceleration. The low initial angular acceleration permits useof a low initial motor torque, that may be expressed as:

τ=Jα

where τ is the motor torque, α is the angular acceleration, and J is themoment of inertia of the load.

Noise is reduced by starting the motor from a stationary position with alower rate of change of torque than would occur with a linear increasein the motor current vector magnitude. The final current vectormagnitude of a motor step must be sufficient to generate the required“torque reserve” that ensures motor startup with a worst-case mechanicalload. A rising exponential current profile permits this final value tobe attained in a given period, i.e., a step of the motor, with a lowerinitial rate of change of current and torque.

The low initial torque reduces power consumption as well as noise.Because torque is a linear function of winding current for operationbelow saturation, only a low initial winding current is required, thatreduces power consumption. The current levels in the initialacceleration steps have the most significant effect on powerconsumption, as these steps are of the longest duration of the group ofsteps. All of the steps contain microsteps of constant period, and theinitial acceleration steps contain the greatest number of microsteps.Use of an exponentially rising acceleration causes the motor to reachits desired maximum speed quickly. The acceleration waveform tablesstored in the memory 22 can be programmed to increase the windingcurrent as the acceleration progresses in order to supply the increasingtorque level required to sustain a nonlinearly rising rate ofacceleration. The tables stored in the memory contain the values for themicrosteps for the step sequences of the waveform for driving the stepmotor.

Furthermore, the controller bandwidth requirement can be minimized byusing microsteps having a constant period. The motor step rate onsucceeding steps during acceleration can be increased by decreasing thenumber of microsteps per motor step and maintaining a constant microstepperiod, rather than by decreasing the microstep period and maintaining aconstant number of microsteps per step. The microstep period is theshortest interval that must be resolved because the microstep determinesthe required bandwidth of the controller. Normally, acceleration iseffected by decreasing this period to achieve a higher rate ofmicrosteps per unit time. Increasing the microstep rate, however,requires an increased controller bandwidth. Keeping the microstep periodconstant during acceleration keeps the controller bandwidth requirementconstant and equal to that for the lowest initial step rate. Since thereis one microstep per motor step at the maximum rate, the motor stepperiod at the maximum rate equals the microstep period at the lowestinitial rate as shown in FIG. 8 where the microsteps are shown on thehorizontal axes and motor steps are shown between the two graphs witharrows surrounding the number of the motor step. Decreasing the numberof microsteps per motor step as the motor accelerates is acceptable,because at higher step rates the rate of change of torque is smoothed bythe motor inductance, and the motor tends to be less sensitive to torquechanges at high speeds.

Once the desired speed for the step motor 10 is reached, a modified fullstep waveform for the maximum constant rate portion of the group ofsteps is used for driving the two-phase step motor 10. A differentwaveform may be desirable for step motors having more than two phases.As shown in FIG. 8, for each step of the modified full step waveform 51and 53, one winding current changes polarity smoothly while the otherremains approximately constant. This is shown in more detail in FIGS.10, 11A and 11B with phase A current 60 and phase B current 62. The rateat which the polarity change occurs is a function of the motor'sinductance and of the motor driver's compliance voltage, as expressedby: $\frac{I_{WINDING}}{t} = \frac{V_{compliance}}{L}$

As V_(compliance) of the controller and the motor inductance, L, areconstant, the current in the winding undergoing polarity reversalchanges approximately as a linear function of time until reaching itsfinal value at the end of the step period. Some nonlinearity may beintroduced by the resistance of the motor winding. The actual fieldvector displacement is a smooth analog function determined by theinductive decay of the motor windings as shown in FIGS. 10, 11A, and11B. The other winding is held at a constant current 66 equal to or lessthan the final value of the decay. The constant current level 66 isselected to minimize power consumption while providing the requiredminimum high speed torque for the specified load.

When the motor is driven at its maximum speed, which is chosen to bewell above its resonance, the acoustic noise normally associated withfull step drive is reduced. Acoustic noise, and power consumption, canbe reduced further by optimizing the constant current level as describedabove to a value that minimizes power consumption while providing therequired minimum high speed torque for the specified load. This resultsin the “modified” full step drive mode. Since one component of thecurrent vector is changing smoothly from initial to final value duringeach high speed step, the resulting field vector displacement (FIG. 11B)is smooth and less noise is produced. There is no increase required inthe controller bandwidth over that of the lowest initial rate drive. Ascan be seen in FIG. 8, each step of the full step waveform 51 and 53 hasa period that preferably equals the period of a microstep.

FIG. 11A shows more detail of part of a waveform of FIG. 10 witharbitrary time increments along the horizontal axis. The phase A current60 can be seen smoothly transitioning polarity within one motor step(eight arbitrary time increments) with resulting lowered noise levels.These arbitrary time units are used once again in FIG. 11B to show thefield vector displacement within the one motor step. Smooth vectorrotation occurs without discrete steps that lead to higher noise levels.Rather than allowing the current to reach the final decay value shown inFIG. 10, the current decay is limited to the constant current level 66so that it takes one motor step for the polarity reversal.

During deceleration, the motor is again driven with a “one phase on”waveform modified for microsteps between each “one phase on” positionwhich begins and ends with the motor in a detent position, where the oneenergized winding or phase at the final step of the group can be turnedon or off without any resulting motor torque. Microstep drive modes areinterspaced with the “one phase on” drive positions to increase torqueand to provide a smooth field vector displacement sequence. No power isrequired to hold the motor in the final stationary detent position forthe group of steps. The permanent magnet in the rotor 12 holds the stepmotor 10 in the detent position. Thus power can be turned off betweengroups of steps to reduce the average power consumption of the stepmotor 10 over a time frame.

As the system progresses through the acceleration, high speed, anddeceleration periods, the processor 20 switches table indices toprogress through the separate tables that contain the acceleration, highspeed, and deceleration waveforms. The number of steps in the timeframe, the total time of the time frame, and the length of the unpoweredinterval are controlled by the processor 20 to precisely determine theaverage step rate of the motor. Certain medical devices, such as fluidpumps, may employ rotation at a selected average rate by grouping stepsto dispense infusates at the proper dosage. For example, see U.S. patentapplication Ser. No. 08/305,677 filed Sep. 12, 1994 to Butterfield etal. entitled System for Increasing Flow Uniformity and incorporatedwherein by reference.

Because the average step rate of the motor is determined by the numberof steps in the step group and the unpowered interval between groups,the same maximum step rate and acceleration-deceleration profile can beused for any desired average step rate, and optimum efficiency can beachieved at all average step rates by selecting an optimally high steprate. The same compliance voltage for the step motor can be used at allaverage rates because the same maximum step rate for the motor is used.Only the number of steps in the group of steps and the unpoweredinterval need be changed to adjust the average rate desired for themotor. The compliance voltage is the maximum voltage required tomaintain a specific value of current over a range of load resistances.The required drive algorithm and hardware are simplified from thatrequired to optimize the efficiency of a motor using a maximum rate stepdrive because the same constant step rate is used to achieve any desiredaverage rate. The same maximum step rate for the waveform, selected foroptimum motor efficiency, is used regardless of the desired average steprate. When using a constant rate step drive, a lower and less efficientconstant step rate must be used in order to attain the desired averagestep rate.

The step motor 10, when driven by the combination of drive modesdescribed above has a lower average power consumption than one driven bya constant rate step drive signal. Low power consumption “one phase on”drive modes are used and no power is consumed during the time separatingthe step groups. The efficiency of the motor is optimized by selecting ahigh maximum step rate whose period matches the required winding currentdecay time determined by motor inductance and compliance voltage toachieve the waveform shown in FIGS. 10, 11A, and 11B.

FIG. 12 is a flow chart illustrating the operation of an open-loopcontrol system in accordance with principles of the invention. Whenmotor movement is to begin 80, the motor accelerates as a result of theexponential application of current to the windings 82. A “one phase on”drive mode modified to microstep between “one step on” positions is usedwith decreasing numbers of microsteps during acceleration 84. When themotor has reached a preselected speed 86, a modified full step drive isused 88. After having rotated the required number of steps at full stepmode and now needing to decelerate 90, a “one phase on” mode modified tomicrostep between the “one phase on” positions with increasing numbersof microsteps for deceleration is used 92. The deceleration is effectedthrough the exponential removal of power to the motor. When the motor isat a detent position 94, power is shut off 96.

While the invention has been illustrated and described in terms ofcertain preferred embodiments, it is clear that the invention can besubject to numerous modifications and adaptations within the ability ofthose skilled in the art. Thus, it should be understood that variouschanges in form, detail and usage of the present invention may be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A control system for controlling the movement ofa motor, the system comprising: an energy source; and a controller thatapplies energy from the energy source to the motor, wherein thecontroller applies energy to the motor in a “one phase on” drive modemodified to microstep between the “one phase on” positions duringmovement of the motor, wherein the controller controls the motor byvarying the number of microsteps per motor step while maintaining aconstant microstep period, and wherein the controller is adapted toapply energy from the energy source to the motor in a non-linear mannerduring acceleration of the motor.
 2. The control system of claim 1wherein the controller is adapted to apply energy from the energy sourceto the motor in an exponential manner during acceleration of the motor.3. A control system for controlling the movement of a motor, the systemcomprising: an energy source; and a controller that applies energy fromthe energy source in a non-linear manner to accelerate the motor throughfirst and second motor steps, wherein the first motor step includes afirst plurality of microsteps, and the second motor step includes asecond plurality of microsteps, and wherein the first plurality ofmicrosteps is greater than the second plurality of microsteps and thecontroller maintains a constant microstep period, wherein the controlleris adapted to apply energy to the motor from the energy source afteracceleration of the motor to a desired speed has been reached with athird number of microsteps per motor step, the controller maintaining aconstant microstep period for each of the third number of microsteps,wherein the controller is adapted apply energy to the motor from theenergy source during deceleration of the motor from the desired speedwith a fourth number of microsteps per motor step, the fourth number ofmicrosteps per motor step exceeding the third number, the controllermaintaining a constant microstep period for each of the fourth number ofmicrosteps, and wherein the controller is adapted to apply energy fromthe energy source to the motor in an exponential manner duringacceleration of the motor.
 4. In an intravenous delivery systemcomprising a fluid supply, a fluid supply line, and a fluid pump havinga fluid pump step motor providing mechanical movement for operating onthe fluid supply line to move fluid through the line, a method fordelivering fluid to a patient, comprising the steps of: accelerating thefluid pump step motor in a series of motor steps, wherein a first ofsaid motor steps is divided into a first number of microsteps, and asecond of said motor steps is divided into a second number ofmicrosteps, wherein the first number of microsteps is greater than thesecond number of microsteps, and maintaining a constant microstepperiod; and applying energy to the motor in an exponential manner duringacceleration of the motor.